The invention relates to a method and device for histogram equalization, and to a display apparatus and a pick-up apparatus comprising such a device.
JP-A-6-90,381 shows a gradation correction device in which a color signal is included as a parameter for gradation correction to obtain a more effective gradation correction. The device is provided with a color parameter calculation circuit weighting for correction to specific color information in addition to gradation correction by a brightness histogram using a luminance signal as a parameter and also with a color parameter correction table deciding a correction coefficient in response to the parameter. When a color signal is inputted and a color of a correction picture element is a color with a visual sensitivity, a correction arithmetic operation circuit increases the quantity of gradation correction. Thus, the dynamic range of a luminance signal is extended in matching with the visual characteristic of the color signal and the gradation is more effectively corrected.
It is, inter alia, an object of the invention to provide an improved color reproduction in case of luminance histogram equalization for colored images. To this end, the invention provides a histogram equalization method and device, as well as a camera and a display apparatus comprising such a device, as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.
In a method of histogram equalization of an image signal in accordance with the present invention, a histogram of the image signal is determined, the image signal is processed in dependence upon the histogram to obtain a modified signal, and the modified signal is enhanced where said modified signal falls below said image signal. Preferably, the enhancement is color saturation dependent.
BE patent 1007609 (Attorneys"" docket PHN 14,585) discloses a color compensation algorithm for the non-linear transfer of the histogram converter which is applied in the histogram conversion IC""s TDA9170, TDA9171 and TDA9178. The goal of this compensation is to restore in the RGB color space the uxe2x80x2vxe2x80x2 color coordinates of each pixel without effecting the luminance of the histogram converter. The uxe2x80x2vxe2x80x2 color space is the top view of the RGB color space of FIG. 2, so a 2D color space that does not take into account the vertical (RGBmax) direction. That color compensation should not be confused with the luminance restoration for rich colored parts as described herein below.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.